1. Field of the Invention
The present invention relates to a transparent thermosensitive recording material, and more particularly to a transparent thermosensitive recording material which forms a colored image upon application of heat thereto and which has high transparency, good image qualities and good ability to be used with thermal printheads.
2. Discussion of the Related Art
A variety of recording materials have been proposed in which a colored image is formed upon application of heat, pressure or the like thereto. Among these recording materials, thermosensitive recording materials are well known which can form a colored image by a chemical coloring reaction when a colorless or a pale-colored coloring agent, such as an electron donor, e.g., a leuco dye having a lactone, a lactam or a spiropyran ring, is brought into contact with a coloring developer, such as an electron acceptor, e.g., an organic acid or a phenolic compound, while heat, pressure or the like is applied thereto.
These thermosensitive recording materials generally include a thermosensitive recording layer (hereinafter referred to as a recording layer) which is formed on a substrate and includes a coloring agent and a coloring developer, and optionally include a protective layer which is formed on the recording layer.
The thermosensitive recording materials have the following advantages over other conventional recording materials:
(1) color images can rapidly be recorded by a relatively simple apparatus without using such complicated steps as developing and fixing; PA1 (2) color images can be recorded without producing noise and environmental pollution; PA1 (3) various color images, e.g., red, blue, violet and black, can easily be obtained; PA1 (4) image density and background whiteness are high; and PA1 (5) the manufacturing cost is low. PA1 (1) sticking occurs in which the surface of the recording layer sticks to the thermal printhead because of the good contact of the recording layer and the thermal printhead, thereby deteriorating image qualities or feeding properties of the thermosensitive recording material; PA1 (2) a part or a component of the recording layer is melted or softened and toughly adhered to the thermal printhead due to the heat applied for recording images, resulting in occurrence of undesirable white streaks in the recorded images because the part of the thermal printhead to which the extraneous substance is adhered has insufficient heat conduction; and PA1 (3) dust tends to be attracted to the surface of the thermosensitive recording material having the plastic film substrate and when a thermal printhead applies heat to the thermosensitive recording material on which the dust is adhered, the dust which tends to stay between the thermal printhead and the highly smoothed surface of the feeding thermosensitive recording material prevents the thermal printhead from contacting the surface of the feeding thermosensitive recording material, resulting in occurrence of undesired white streaks in the recorded images.
Because of these advantages, the above-described thermosensitive recording materials can be widely used, not only as a recording material for price labels in stores, but also as recording materials for copiers, printers for computers, facsimiles, automatic vending machines of labels and tickets, video printers and measuring instruments.
Recently, in the medical field or the like, a transparent recording material which can easily produce an image using a dry developing process without environmental pollution has become desired since photographic recording materials using silver halide which have been used as recording materials for radiography have waste-disposal problems of the developing liquid. In addition, a transparent recording material is also desired which can easily produce an image by a method using digital signal image processing which has become widely employed in recent years. Thermal recording is suitable as a recording process for radiography because of having the above-mentioned advantages; therefore, a transparent thermosensitive recording material is needed to serve as transparent print output sheets for radiography printers or the like.
In thermal recording processes, thermal printheads have been widely used as a heating medium. The thermal printhead applies heat to a thermosensitive recording material while contacting the surface of the thermosensitive recording material. When the surface of the recording layer of a thermosensitive recording material having high smoothness (because, for example, the recording layer is formed on a film substrate) is heated with a thermal printhead to record an image, the following problems tend to occur:
In attempting to solve these problems, i.e., sticking and white streaks, a thermosensitive recording material having a protective layer which includes a filler having a predetermined particle diameter has been disclosed (Japanese Laid-Open Patent Application No. 6-166265). However, this thermosensitive recording material has poor transparency and therefore cannot be used for purposes for which a transparent recording material is desired. Japanese Laid-Open Patent Application No. 7-76168 discloses a transparent thermosensitive recording material whose surface has a predetermined degree of smoothness. However, the latter transparent thermosensitive recording material does not have good ability to be used with thermal printheads.
Because of these reasons, a need exists for a transparent thermosensitive recording material having high transparency, good image qualities without white streaks and good ability to be used with thermal printheads without sticking to the thermal printheads.